The present invention relates to a method of genetically evaluating animals by assaying for the presence of at least one genetic marker which is indicative of one or more of the traits of fat content, growth rate, and feed consumption. In particular, the method analyzes for variation in the melanocortin-4 receptor (MC4R) gene which is indicative of these traits. Even more particularly, the method analyzes for a polymorphism in the MC4R gene.
There is an increasing consumer demand for meat products having low fat content. This demand is fueled by accumulating evidence in the scientific literature that a high consumption of animal fat, especially fat with a high proportion of saturated fatty acids, represents a significant health hazard, including risk for cardiovascular disease. Other health concerns associated with high fat meats include their high content of cholesterol and the addition of relatively high amounts of salt which are added to improve the binding characteristics since salt aids in extracting the native water binding component myosin from the meat. Furthermore, an increasing number of consumers find meat products containing chemical additives such as phosphates, emulsifying additives, and anti-oxidants less acceptable.
Faced with consumers who seek a healthier meat product, meat producers are continually pressed to offer cheaper and healthier products.
Cheaper products, of course, come from lowering costs of production. Producers are always interested in improving the growth rate and feed conversion of their animals. Lower production costs come from the shorter time to market and lower costs of feeding an animal. This can increase the profit margin in the livestock industry and/or result in lower prices to the consumer.
By being able to select for animals which have the aforementioned traits, producers can raise animals with these desirable characteristics. Selection for desirable traits has traditionally been done using breeding techniques.
Genetic differences exist among individual meat producing animals as well as among breeds which can be exploited by breeding techniques to achieve animals with these desirable characteristics. For example, Chinese breeds are known for reaching puberty at an early age and for their large litter size, while American breeds are known for their greater growth rates and leanness. Thus, it would be desirable to combine the best characteristics of both types of these breeds, thereby improving pork production.
Often, however, heritability for desired traits is low, for example, heritability for litter size is around 10%-15%. Standard breeding methods which select individuals based upon phenotypic variations do not take fully into account genetic variability or complex gene interactions which exist. Therefore, there is a need for an approach that deals with selection for leanness, growth rate, and feed consumption at the cellular or DNA level. This method will provide a method for genetically evaluating animals to enable breeders to more accurately select those animals which not only phenotypically express desirable traits but those which express favorable underlying genetic criteria. This has largely been accomplished to date by marker assisted selection.
Restriction fragment length polymorphism (RFLP) analysis has been used by several groups to study pig DNA. Jung et al., Theor. Appl. Genet., 77:271-274 (1989), incorporated herein by reference, discloses the use of RFLP techniques to show genetic variability between two pig breeds. Polymorphism was demonstrated for swine leukocyte antigen (SLA) Class I genes in these breeds. Hoganson et al., Abstract for Annual Meeting of Midwestern Section of the American Society of Animal Science, Mar. 26-28, 1990, incorporated herein by reference, reports on the polymorphism of swine major histocompatibility complex (MHC) genes for Chinese pigs, also demonstrated by RFLP analysis. Jung et al., Animal Genetics, 26:79-91 (1989), incorporated herein by reference, reports on RFLP analysis of SLA Class I genes in certain boars. The authors state that the results suggest that there may be an association between swine SLA/MIHC Class I genes and production and performance traits. They further state that the use of SLA Class I restriction fragments, as genetic markers, may have potential in the future for improving pig growth performance.
The ability to follow a specific favorable genetic allele involves a novel and lengthy process of the identification of a DNA molecular marker for a major effect gene. The marker may be linked to a single gene with a major effect or linked to a number of genes with additive effects. DNA markers have several advantages; segregation is easy to measure and is unambiguous, and DNA markers are co-dominant, i.e., heterozygous and homozygous animals can be distinctively identified. Once a marker system is established selection decisions could be made very easily, since DNA markers can be assayed any time after a tissue or blood sample can be collected from the individual infant animal.
The use of genetic differences in receptor genes has become a valuable marker system for selection. For example, U.S. Pat. Nos. 5,550,024 and 5,374,526 issued to Rothschild et al. disclose a polymorphism in the pig estrogen receptor gene which is associated with larger litter size, the disclosure of which is incorporated herein by reference. U.S. application Ser. No. 08/812,208 discloses polymorphic markers in the pig prolactin receptor gene which are associated with larger litter size and overall reproductive efficiency.
It can be seen from the foregoing that a need exists for a method for selecting animals with the improved metabolic traits regarding fat content, growth rate, and feed consumption.
An object of the present invention is to provide a genetic marker based on or within the MC4R gene which is indicative of fat content, growth rate, and/or feed consumption.
Another object of the invention is to provide an assay for determining the presence of this genetic marker.
A further object of the invention is to provide a method of evaluating animals that increases accuracy of selection and breeding methods for the desired traits.
Yet another object of the invention is to provide a PCR amplification test which will greatly expedite the determination of presence of the marker.
An additional object of the invention is to provide a kit for evaluating a sample of animal DNA for the identified genetic marker.
These and other objects, features, and advantages will become apparent after review of the following description and claims of the invention which follow.
This invention relates to the discovery of a polymorphism within the melanocortin-4 receptor (MC4R) gene which is associated with fat content, growth rate, and feed conversion traits in animals. According to the invention, the association of the MC4R polymorphism with the trait(s) enables genetic markers to be identified for specific breeds or genetic lines. The TaqI restriction pattern which identifies the polymorphism is used to assay for the presence or absence of markers associated with the desirable metabolic traits. The breed-dependent marker genotype (i.e., a marker in some breeds and a nonmarker in others) consists of a polymorphism within MC4R, a guanine to adenine transition at position 678 of the PCR product (a missense mutation of aspartic acid codon (GAU) into asparagine codon (AAU) at position 298 amino acid of the MC4R protein). The invention includes assays for detection of the marker as well as the sequence characterization of the polymorphism and includes novel sequences in the MC4R gene which may be used to design amplification primers for such an assay. Additionally, the invention includes a method for using the assay in breeding programs for animal selection and a kit for performing the assay.
As used herein, xe2x80x9clow fat contentxe2x80x9d or xe2x80x9cleannessxe2x80x9d means a biologically significant decrease in body fat relative to the mean of a given population.